MEAN is composed of four technologies :
Express is a minimalistic framework that is used to write mature, flexible, lightweight server code with few lines of code. It makes a node framework broader providing support for different templating engines like EJS, handlebars and many more using the Pug engine. All native features of the node are optimized and are much easier to obtain with the help of this framework.
AngularJS allows us to use HTML as a template language. Therefore, we can extend HTML's syntax to express the components of our application. Angular features like dependency injection and data binding eliminate plenty of code that you need to write.
It also allows developers to create data-intensive real-time apps that run across distributed devices. We can run Node.js runtime on Microsoft Windows, OS X, and Linux.
Each component involved in MEAN Stack has some dedicated purpose in the application.
Angular JS: Since it is a front-end JS framework, it's main purpose is to accept requests and display results to end-user.
NodeJS: Since it is a JS back-end framework, it is used to handle all client's requests and process them.
Express JS: It is used to make requests to the database(basically MongoDB) and return back response.
MongoDB: Its main purpose is to store data in JSON-like format and retrieve them.